410 



he proceeds to show its litliological and geological identity. This 

 limestone is described in detail at the Castle Hill, Wren's Nest, and 

 Hurst Hill, in all of vvhicli it forms ellipsoidal elevated masses, 500 to 

 650 feet high, protruding through the coal measures in lines paral- 

 lel to similarly shaped masses of Ludlow rock at Sedgeleyj &c., i. e. 

 trending from 10° E. of N. to 10° W, of S. Two strong bands of 

 limestone occur in these hills, overlaid and separated from each other 

 by shale, charged with numerous small concretions of impure lime- 

 stone, the "bavin" of the workmen. The limestone having been quar- 

 ried out from these bands which have been raised up from a common 

 centre, and disposed with a quaquaversal dip at high angles, it is evi- 

 dent that the hills themselves would ere now have been demolished, 

 had they been composed throughout of calcareous masses of equal 

 purity; but the " bavin" or refuse composes the framework of these 

 perforated hills, and preserves their outline. The Wenlock shale, or 

 underlying part of the formation, constitutes the nucleus of the 

 Wren's Nest, the largest and most perfect of these ellipsoids, and 

 of this the author gives a detailed plan. These ellipsoids usually 

 feather off at one extremity with a broken-down margin, and thus 

 complete their resemblance in physical features to ancient craters of 

 eruption*. The greatest superficial extent of the Wenlock formation 

 is in the neighbourhood of Walsall, where it rises both in dome- 

 shaped masses and in rectilinear ridges, running from S.S.W. to 

 N.N.E. parallel to the axis of the Wolverhampton coal-field, of which 

 one of these ridges forms the eastern boundary, the limestone 

 plunging beneath the coal-field at a rapid angle. The oiher ridge is 

 continuous with the new red sandstone of the Bar-beacon, and is 

 known as the Hay Head lime. In the Dudley or 1 0-yard coal tract 

 few works have yet proceeded downwards beneath the lower coals, 

 and hence the subjacent Silurian rocks are little known to the miners. 

 A remarkable and accidental discovery of a mass of limestone took 

 place recently near Dudley Port, on the rise side of a great fault, which 

 bounds the downcasts of the coal, called " Dudley Trough." Having 

 worked out the coal on the upcast side, a shaft was sunk in and upon 

 the southern side of this fault, when at a depth of 208 yards, and about 

 100 yards below the exhausted coal strata, a mass of limestone was 

 met with, which proved to be near 7 yards thick, and of very good 

 crystalline quality. Being found to extend in a form more or less ho- 

 rizontal, extensive works were promptly opened in it for the extrac- 

 tion of a rock so precious in the heart of the coal-field. When the 

 author visited it, a considerable caviry had been formed, in which no 

 trace of moisture was discernible, whilst it was known that copious 

 streams of water were flowing in the coal measures overhead. He 

 accounts for this mass of limestone being hermetically excluded from 

 the percolation of water, by the impervious nature of the Silurian shale 

 which separates the coal measures from the limestone, and by the shafts 

 being sunk in the fault itself, which, like other lines of fissure, is filled 

 up with clay and other materials, so closely compacted as to form com- 



* Sec account of Valley of Woolhope for similar phenomena on a larger 

 scale, and with a greater number of concentric and enveloping formations. 

 — Proceedings Geol. Soc, vol. ii. p. 15. 



